Why must you alternate plus and minus when inserting batteries?

August 10, 2010

Dear Straight Dope:

Why do electronic devices require you to insert batteries so that you alternately have the plus and minus ends on top? In other words, why do you never see (except in a flashlight) all of the "+" signs at the top and the "–" signs at the bottom or vice versa?

— Brad McKown, Orlando, Florida

SDStaff Rico replies:

It's to keep wiring to a minimum. Get ready for Batteries 101.

Let's assume we're talking about standard alkaline batteries, the kind you buy at the supermarket. In dealing with batteries (or anything having to do with electricity), you need to think about two things: the voltage,expressed in volts (duh), and the current, expressed in amperes or amps.

If you think of electricity as water flowing in a hose, the voltage is the pressure — this determines how far the water will squirt, or how bright the light will get. The current is the volume. The wider you open the hose nozzle, the more water you can deliver, and the wetter your lawn will get. In a flashlight, the limiting factor is the bulb — the bigger the bulb, the more current it requires. For the sake of simplicity, let's assume that the typical flashlight bulb draws 1 ampere.

The voltage of a single battery (technically a cell) is 1.5 volts. The discharge rate of batteries — that is, their capacity, or how long they'll last — is expressed in ampere-hours. For example, a AA cell has a discharge rate of 2.85 ampere-hours, meaning it can deliver 1.5 volt at 1 ampere for 2.85 hours before it's kaput.

Suppose you're designing a flashlight that requires 6 volts of power and draws 1 ampere of current. How will you get 6 volts out of standard AA cells?

Simple. You connect them in series — that is, you stack them nose to tail, as shown in the diagram below. This adds up the voltages (1.5 volts × 4 cells = 6 volts).

Note that the amperage remains the same. So 4 AA cells in series will provide 6 volts at 1 ampere for 2.85 hours.

In a typical battery case, where the cells are inserted so that they're alternately positive side up/negative side up, in effect you're folding the cells so the negative side of each is wired to the positive side of the next. This saves wire and makes for a more compact arrangement, but it's electrically equivalent to placing the batteries in a line:

Now let's throw a wrench in the works: You're designing a flashlight that only requires 1.5 volts and draws one ampere of current, but it must operate for a minimum of 10 hours. What do you do? Easy. You connect the 4 AA cells in parallel to add up the ampere-hours (2.85 ah × 4 = 11.4ah), like so:

So you've got enough power for 11 hours plus change. Piece of cake.

What would happen if you reversed the cells randomly? Why, you'd rip a huge tear in the space-time continuum and we'll all be sucked into the black hole called …

(AAAAAAAAaaaaaaaaaaaaah!)

Disclaimer: The above is a simplified explanation of battery life. If we really got into this topic, we'd compare 6 hour vs. 20 hour vs. 100 hour discharge rates, the type and composition of the batteries used, and different discharge scenarios. But we won't.

— SDStaff Rico

Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.